90 research outputs found
CO adsorption on Pt induced Ge nanowires
Using density functional theory, we investigate the possible adsorption sites
of CO molecules on the recently discovered Pt induced Ge nanowires on Ge(001).
Calculated STM images are compared to experimental STM images to identify the
experimentally observed adsorption sites. The CO molecules are found to adsorb
preferably onto the Pt atoms between the Ge nanowire dimer segments. This
adsorption site places the CO in between two nanowire dimers, pushing them
outward, blocking the nearest equivalent adsorption sites. This explains the
observed long-range repulsive interaction between CO molecules on these Pt
induced nanowires.Comment: 12 pages, 10 figure
Models and Simulations in Material Science: Two Cases Without Error Bars
We discuss two research projects in material science in which the results
cannot be stated with an estimation of the error: a spectro- scopic
ellipsometry study aimed at determining the orientation of DNA molecules on
diamond and a scanning tunneling microscopy study of platinum-induced nanowires
on germanium. To investigate the reliability of the results, we apply ideas
from the philosophy of models in science. Even if the studies had reported an
error value, the trustworthiness of the result would not depend on that value
alone.Comment: 20 pages, 2 figure
Rationality: a social-epistemology perspective
Both in philosophy and in psychology, human rationality has traditionally been studied from an "individualistic" perspective. Recently, social epistemologists have drawn attention to the fact that epistemic interactions among agents also give rise to important questions concerning rationality. In previous work, we have used a formal model to assess the risk that a particular type of social-epistemic interactions lead agents with initially consistent belief states into inconsistent belief states. Here, we continue this work by investigating the dynamics to which these interactions may give rise in the population as a whole
Formation of Pt induced Ge atomic nanowires on Pt/Ge(001): a DFT study
Pt deposited onto a Ge(001) surface gives rise to the spontaneous formation
of atomic nanowires on a mixed Pt-Ge surface after high temperature annealing.
We study possible structures of the mixed surface and the nanowires by total
energy (density functional theory) calculations. Experimental scanning
tunneling microscopy images are compared to the calculated local densities of
states. On the basis of this comparison and the stability of the structures, we
conclude that the formation of nanowires is driven by an increased
concentration of Pt atoms in the Ge surface layers. Surprisingly, the atomic
nanowires consist of Ge instead of Pt atoms.Comment: 4 pages, 3 figure
The Formation of Self-Assembled Nanowire Arrays on Ge(001): a DFT Study of Pt Induced Nanowire Arrays
Nanowire (NW) arrays form spontaneously after high temperature annealing of a
submonolayer deposition of Pt on a Ge(001) surface. These NWs are a single atom
wide, with a length limited only by the underlying beta-terrace to which they
are uniquely connected. Using ab-initio density functional theory (DFT)
calculations we study possible geometries of the NWs and substrate. Direct
comparison to experiment is made via calculated scanning tunneling microscope
(STM) images. Based on these images, geometries for the beta-terrace and the
NWs are identified, and a formation path for the nanowires as function of
increasing local Pt density is presented. We show the beta-terrace to be a
dimer row surface reconstruction with a checkerboard pattern of Ge-Ge and Pt-Ge
dimers. Most remarkably, comparison of calculated to experimental STM images
shows the NWs to consist of germanium atoms embedded in the Pt-lined troughs of
the underlying surface, contrary to what was assumed previously in experiments.Comment: 6 pages, 4 figures, E-proceedings of 2009 MRS spring Meetin
Assigning probabilities to non-Lipschitz mechanical systems
We present a method for assigning probabilities to the solutions of initial
value problems that have a Lipschitz singularity. To illustrate the method, we
focus on the following toy-example: , , and
, where the dots indicate derivatives to time and
. This example has a physical interpretation as a mass in a
uniform gravitational field on a dome of particular shape; the case with
is known as Norton's dome. Our approach is based on (1) finite
difference equations, which are deterministic, (2) a uniform prior on the phase
space, and (3) non-standard analysis, which involves infinitesimals and which
is conceptually close to numerical methods from physical praxis. This allows us
to assign probabilities to the solutions of the initial value problem in the
original, indeterministic model.Comment: 13 figure
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